On Aquatic Carnworous Coleoptera or Dytiscide. 217 
tance, and is more or less distinctly marked off into three parts, 1. The portion 
behind the coxe, this is called the prosternal process ; 2. The portion lying between 
the coxee, which may be called the intercoxal band ; and 3. The portion in front 
of the coxze, which may be called the anterior band. This anterior band forms on 
either side the front boundary of the coxal cavity, and at the outer side of the coxa 
is prolonged backwards, and articulates by a very distinct suture with the posterior 
side-piece or epimeron ; external to the coxa the anterior band is continued outwards, 
forming the side wing of the prosternum proper ; this side wing is always very 
short (in the longitudinal direction). The anterior band varies somewhat in length, 
according to whether the front coxe reach very near to the anterior edge of the 
prosternum or not ; it is extremely short in Hyphydrus but is considerably longer 
in most of the Noterides, and in the Colymbetides, Dytiscini, Cybistrini and Hyda- 
ticides it is of moderate length (except in Eretes where it is exceptionally short), 
and in these groups, which might be called the Macro-Dytiscid, in consequence of 
their greater size contrasted with the other groups, it is thickened along the middle, 
though to a variable extent ; this thickening causes the undersurface of the prothorax 
to be of a keel-like form, as may be well seen in the Cybistrini where it reaches 
its greatest degree of development ; in the Colymbetides this keel-like 
thickening is usually much less than it is in the Cybistrini and Hydaticides, 
and in the lower forms of the Agabini such as Agabus cephalotes (No. 663) and 
A. cordatus it may be seen in its rudimentary condition ; Coptotomus, however, is 
an exception in this respect to its allies. Passing now to the intercoxal portion of 
the prosternum, we find that in the Hydroporides this part forms a slender band 
between the cox, and that it attains its greatest elongation in Hyphydrus ; in 
some of the Bidessini (Pachydrus), a very peculiar condition of this band exists, in 
front its margins are a little raised so that it is somewhat depressed or sulcate 
along the middle, but about halfway of its length it becomes suddenly very promi- 
nent or protuberant, and convex instead of suleate, giving rise to the suggestion 
that the anterior and posterior portions are distinct pieces ; traces of this division 
ot the intercoxal band into two portions exist innumerous members of the Hydro- 
porides, inasmuch as the posterior portion is less flat transversely than the anterior, 
and that there isa more or less distinct tubercular prominence at the junction of 
the two parts (vide Dytiscus parallelogrammus, No. 416, and numerous other 
species of Celambus and the allied genera). In the Macro-Dytiscide the condition 
of the prosternum between the coxee is very different, for the thickening or incrass- 
ation of the middle which I have described when speaking of the anterior band, is 
continued between the coxe, go that here the prosternum projects beyond the level 
of the coxee, or appears as it were to be arched over them, not merely produced 
between them, as in the Hydroporides. Behind the cox, the prosternum becomes 
broader, though sometimes only slightly so (Cybister, Dytiscus, and many 
others), and is more or less prolonged backwards to form the prosternal 
